The present invention relates to control devices for internal combustion engines and particularly small engines. The usual type of controller is a PID controller in which a signal proportional to engine RPM is combined with a derivative signal and an integral signal to generate an error voltage used to regulate a throttle or other engine speed control device.
With particular reference to smaller engines, there is a greater potential for damage to the engine in the event that there is a failure in the control electronics. Indeed, there are several types of failures to be dealt with in order to fully protect the engine from damage. For example, if the magnetic pick up device used to monitor engine RPM, fails, an ordinary speed governor may interpret the absence of such signals as a below set point speed error and, therefore, continually increase engine speed until damage occurs. In reality, such a condition may be nothing more than a broken pickup coil or associated problem. It is necessary, therefore, to provide a fail safe circuit which will not respond in this manner in the absence of magnetic pickup signals.
Similarly, in the event that a governor output transistor becomes shorted, the governor loses its ability to shut down the engine and, again, over speed conditions can occur which will cause damage. A third problem frequently encountered is damage to the potentiometer by which the desired speed reference value is set. If the potentiometer fails, the governor would be provided with an incorrect reference voltage also leading to the engine's damage.
Accordingly, it is desired to provide an improved speed governor circuit and monitor circuit whereby the failures described can be detected and appropriate action taken to prevent damage to the engine.
A further object is to provide circuitry which will automatically employ a percentage of the engine set RPM for the overspeed monitor threshold value.
It is a further object of the invention to provide a PID governor which can have a low gain during normal operation but which provides high gain during start up to prevent saturation of the integrator portion of the governor circuit.
These and other objects and advantages of the invention will be apparent from the remaining portion of the specification.